Over the past twenty-five years or so, the primary challenge of very large scale integration (VLSI) has been the integration of an ever-increasing number of metal oxide semiconductor field effect transistor (MOSFET) devices with high yield and reliability. This was achieved mainly in the prior art by scaling down the MOSFET channel length without excessive short-channel effects. As is known to those skilled in the art, short-channel effects are the decrease of threshold voltage Vt in short-channel devices due to two-dimensional electrostatic charge sharing between the gate and the source/drain diffusion regions.
In recent years, and as channel lengths are being scaled below 0.1 μm, SOI complementary metal oxide semiconductor (CMOS) technology has received considerable interest in VLSI for its potential low-voltage, low-power, and high-speed advantages in comparison to bulk CMOS devices. As known to those skilled in the art, SOI structures include an insulating layer, i.e., buried oxide region (BOX), that electrically isolates a top Si-containing layer from a bottom Si-containing layer. The top Si-containing, i.e., the SOI layer, serves as the area in which electronic devices such as MOSFETs can be fabricated.
Thin film SOI MOSFETs in which the top Si-containing layer has a thickness of about 20 nm or less are of special interest due to improved isolation, reduced parasitic capacitance as well as the reduction of short-channel and floating body effects that can be obtained from such technology. Despite the known advantages with thin film SOI technology, processing challenges exist which substantially hamper the use of thin film SOI MOSFETs in semiconductor integrated circuits. For example, prior art processes for fabricating thin film SOI MOSFETs have difficulty in forming a thin (20 nm or less) SOI channel region, while simultaneously being able to maintain abutting thick SOI source and drain regions. Thick source and drain regions are desirable since they permit the formation of a low sheet resistance silicide layer.
In view of the above-mentioned drawbacks with fabricating prior art thin SOI MOSFETs, there exists a need for providing a new and improved method for fabricating SOI MOSFETs which have a thin SOI device channel region, i.e., a recessed channel, as well as thick SOI source and drain regions abutting the thin channel region.